An industrial robot comprises a manipulator and a control system. The manipulator comprises links movable relative to each other. The links are different robot parts such as a base, arms, and a wrist. Each joint has joint components such as a motor, motor gears and motor bearings. Actuators, such as motors, drive the movements of the manipulator. The control system comprises one or more computers and drive units for controlling the manipulator. The position and speed of the links are controlled by the control system of the robot that generates control signals to the motors. The control system includes a path planner adapted to calculate reference values for the position of the actuators. The drive units are adapted to calculate reference values for the torques of the actuators. The drive unit is provided with a feedback loop calculating a feedback torque for the actuator based on measured position values from the actuator and the position reference values from the path planner. The drive units are adapted to calculate the reference values for the torques of the actuators based on the feedback torque from the feedback loop. In order to improve the control of the actuators, some drive units are further provided with a feed forward loop calculating a feed forward torque for the actuator based on the position reference values from the path planner and a mathematical model of the robot. In those cases, the control units are adapted to calculate the reference values for the torques of the actuators based on the feedback torque from the feedback loop as well as on the feed forward torque from the feed forward loop.
Industrial robots are used in industrial and commercial applications to perform precise and repetitive movements. It is then important for a faultless functionality of the robot that the industrial robot is performing according to its nominal performance, which means that the links have to be in good condition and perform together in an expected way.
However, it is difficult to detect or determine if an industrial robot is not performing according to its nominal performance. The operator, such as a service technician, has to rely on what he sees and on information from the control system about the performance of the robot, such as the position and speed of the motors taken from readings on sensors on the manipulator. The operator then analyses the present condition of the robot based on his personal experience resulting in a varying diagnosis due to subjective measures. In many cases the operator analysing the present condition and performance of the robot also needs to evaluate information from different sources, such as different motors at the same time or external conditions in the facility where the robot is located or is even faced with an emergency stop. To find the cause of a failure the operator may have to try different hypotheses and it is therefore time consuming and often results in long stand-still periods for the robot causing huge costs.
Due to frequent personal rotation today, operators of robot service technician staff do not have sufficient experience to diagnose and isolate a failure in performance of the robot.
It is desirable to attain a simple method to diagnostic the present performance or condition of the robot.
JP62024305 discloses a method for detecting servo abnormality of an industrial robot. The method includes monitoring the deviation between the reference value of the position of an actuator of a drive source and present values of the position of the actuator and based thereon detect servo abnormality. This method supervises the control error, i.e. the difference between the reference position and the measured value of the position in order to detect abnormalities. A disadvantage with this method is that the control error has large natural variations, for example due to changes in the position reference values, which are not caused by an error in the robot performance. Abnormalities due to wear of the drive components, such as the motors, motor gears, motor bearings and axes, and brakes, causes small and slow changes in the control error. Accordingly, it is difficult to detect changes in the control error due to wear in the drive components.